Electrophotographic apparatus with improved toner transfer

ABSTRACT

The invention concerns electrophotographic apparatus provided with an optical photoconductive (OPC) conveying means charged with an image pattern of toner and improved transfer means including an insulating transfer member for effecting a more positive transfer of toner from the OPC to a copy sheet. In one form, the additional transfer member is a Mylar* (*Trademark of Du Pont &amp; Co.) flap partially covering the transfer corona station and, in another case, a nylon or Dacron* mesh covering the entire transfer station. A negative potential is established on the transfer corona wires to charge the paper for effecting transfer of toner and, at the same time, to charge the additional transfer member to repel the paper and insure a closer contact with the OPC material.

United States Patent [72] Inventors MichaelJ. Kelly;

Myron F. Shlatz, both of Lexington, Ky. [21] Appl. No. 879,073 [22] Filed Nov. 24. 1969 [45] Patented Nov. 16,1971 [73] Assignee International Business Machines Corporatitm Armonk, N.Y.

[54] ELECTROPIIOTOGRAPIIIC APPARATUS WITII IMPROVED TONER TRANSFER 8 Claims, 15 Drawing Figs.

[52] US. Cl 355/3, 96/l.4, 250/495 C, 118/637 511 Int. Cl. G03g 15/00 [50] Field ofSearch 355/3, 7, l7; 317/4, 262; 96/l.4; 118/623, 624, 625,637; 250/495 2 [56] References Cited UNITED STATES PATENTS 2,777,957 1/1957 Walkup 250/495 ZC Primary Examiner-John M Horan Assistant Examiner-Mathews Allan Atwrneys- Hanifin and .lancin and D. Kendall Cooper ABSTRACT: The invention concerns electrophotographic apparatus provided with an optical photoconductive (OPC conveying means charged with an image pattern of toner and improved transfer means including an insulating transfer member for effecting a more positive transfer of toner from the OPC to a copy sheet. In one form, the additional transfer member is a Mylar (Trademark of Du Pont & Co.) flap partially covering the transfer corona station and, in another case, a nylon or Dacron mesh covering the entire transfer station. A negative potential is established on the transfer corona wires to charge the paper for effecting transfer of toner and, at the same time, to charge the additional transfer member to repel the paper and insure a closer contact with the OPC material.

BACKGROUND OF THE INVENTION Electrophotography using photoconductive insulating layers upon which an electrostatic image is formed, for example, as is described in U.S. Pat. No. 2,297,691, has become embodied in a number of high-speed copying processes. The photoconductive insulating layer is backed by a conductive layer and can be formed in the shape of a cylinder which is then rotated to. bring the electrophotographic element to a number of stations involved in carrying out the electrophotographic process. The photoconductive insulating layer is first charged. by applying an electrical potential acrossit. The charged, photoconductive layer is then exposed imagewiseto light and the electrical potential decays in the surface areas which are struck by light. The dark.areas'of theprojected image retain their electrostatic charge, and the image is then developed by exposing the surface of the photoconductive layer to small charged marking particles known as toner particles. The charged toner particles are attracted to the charged image areas of the photoconductive layer and thereby develop the electrostatic image. The image .can then be transferred from the photoconductive layer to a copy sheet.

A number of ways are conventionally employed to develop the electrostatic image as is well known in the art. These include cascade development described, for example, in U.S. Pat. No. 2,618,552; powder cloud development described, for example, in U.S. Pat. No. 2,221,776; magnetic brush development described, for example, in U.S. Pat. No. 2,874,063; fur brush development, donor belt development, impression development and liquid spray development. Commonly used processes in commercial copying machines are cascade and magnetic brush development. These methods employ toner particles usually comprising a heat softenable resin binder material, for example, a natural or synthetic organic compound or polymer such as styrene polymers and copolymers, epoxy resins, rosin, rosin esters, polymers of acrylic and methacrylic acid esters such as those prepared by polymerizing such monomers as methyl methacrylate, butyl methacrylate, ethyl acrylate and mixtures thereof, Various physical and chemical combinations of such polymers can also be employed. The resins are mixed with coloring matter, for example, carbon black so that a colored image can be easilyheat fused onto a copy sheet. Other additives, such as plasticizers and anticaking agents can also be employed in the toner composition.

After development, the image is transferred from the photoconductive layer to the copy sheet, such as plain paper. conventionally, the paper is placed in contact with the developed image on the element and the toner is transferred by an electrical charge and/or mechanical pressure applied to the paper. The paper carrying the toner image is then stripped from the photoconductor.

In prior devices, a transfer corona station is provided which has a relatively high negative potential applied to associated corona wires that charge the paper and enable the transfer of the toner image from the photoconductor element to the paper.

The toner and paper are than carried to a fuser where the toner is fused to the paper by heat. The reproduction is carried to the output pocket and stored there.

In prior devices, it has been observed that the conventional transfer corona station does not always operate in the most efficient manner; and it has not been found possible to make a complete transfer of toner in many cases to the copy sheet. In many cases in prior devices, vertical streaks and dropout occurs in the transfer process reducing the quality of the finished copy.

Because it has not been found possible to make a complete transfer of toner to the copy sheet, a portion of the toner image remains on the photoconductor surface and small amounts of finely divided toner particles also cling to the nonimage areas. If not removed, this residual image and background toner will appear on the following copies. Therefore, it is necessary to provide a cleaning station for cleaning the surface of the photoconductor prior to the next copying cycle.

The photoconductor, which has some residual toner on it, is charged by a positive corona discharge device and then brushed to remove the majority of residual toner. The

- photoconductor is then ready for another reproduction of the same document or is then brushed again before anew document is inserted in the machine.

A number ofways of removing residual toner particles have been employed such as, for example, by physically contacting the surface of the photoconductofwith either a renewable web of fiber material or with the bristles of a rotating cylindrical brush-and, also, by contacting the photoconductor surface with high velocity air. The removed particles are carried away from the cleaning station to a' suitable receptacle by providing an air pressure differential. Light and/or'corona discharges can be employed to neutralize a portion of thecharge on the photoconductor prior to the time that it reaches thecleaning station sothat the particles are more easily removed.

The present arrangement improves transfer of toner from the photoconductor to thepaper. This leaves less residual toner on the photoconductor which is beneficial since the filming rate may be reduced and the cleaning bag life may be increased ona per cycle basis. 7 I

A lower amountof toner is likely to be used due to the fact that a lower toner concentration produces the same optical density of the characters. This is a direct function of the improved transfer efficiency realized.

BRIEF SUMMARY OF THE INVENTION The invention concerns electrophotographic apparatus having provision for an additional transfer member associated with a transfer corona station in a particular manner to improve the transfer of toner from the OPC element to'a copy sheet. This is effected, in one case, by providing a flap member partially covering the transfer station or, in another case, by a mesh member that covers the entire station with the objective of providing some open space for the charge field normally established at the transfer station. A supply source ordinarily establishes a relatively high negative potential on transfer corona wiresatthe transfer station. This same source is used to establish-a negative potential on the flap or the mesh, as the case may be. Thisnegative potential repels the negative potential on th'ecopy sheet to establish pressures insuring that the copy sheet is in better contact with the OPC element. In one case, the 'flap element is comprised of a Mylar material; and in the other case, themesh is formed of nylon or similar material. Also, the additional element tends to establish a higher negative charge on the reverse side of the copy paper, tending to attract the positively charged toner from the OPC to the paper in a more efficient fashion. In using the nylon mesh, it is preferable that the mesh be oriented with the crossingthreads arranged at a 45 angle to the path of movement of the OPC element and copy paper. It has also been found that a Dacron mesh is quite suitable in the practice of the present invention. it is evident that some other angular relationship may be appropriate. A range of 20 to 70 for the'mesh could possibly be satisfactory.

A particular advantage occurs when out sheets are used rather than a roll or continuous web of paper, since it is difficult to maintain contact pressure with sheets. in our case, the trailing edge of sheets presents a problem which this approach helps.

OBJECTS A primary object of the present invention is to improve toner transfer from an OPC element to a copy sheet in an electrophotographic apparatus.

Another object of the present invention is to establish higher contact conditions at a transfer corona station for effecting a more efficient transfer of toner.

Another object of the invention is to reduce the residual toner on the OPC element after transfer thereby possibly reducing filming rate and increasing filter bag life.

A further object of the invention is to reduce the total amount of toner used in the system due to increased transfer efficiency, lowering the toner concentration necessary for acceptable copy density.

A particular object of the invention is to enhance the transfer of toner from a photoconductor element to paper to avoid streaking, dropout conditions, and improve overall quality of the copy.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of various preferred embodiments of the invention as illustrated in the accompanying drawings.

DRAWINGS FIG. 1 is a system diagram of an electrophotographic copying machine incorporating the transfer corona improvements 2 according to the present invention.

FIGS. 2a, 2b and 2c illustrate a prior transfer corona unit that is useful in explaining some of the problems encountered with such prior devices.

FIGS. 3a, 3b, and 30 represent a prior copy transfer condition showing streaking and dropout of toner, a transfer using a Mylar flap, and a transfer using a Dacron mesh, respectively.

FIGS. 40 and 4b illustrate a possible grid mat configuration i that is usable in practicing the present invention.

FIGS. 5a, 5b, and 5c illustrate a transfer corona station incorporating a Mylar flap in a particular embodiment of the invention.

FIGS. 60, 6b, and 6c illustrate a transfer corona device similar to that shown in FIGS. 5a, 5b, and 50, with the exception that a Dacron mesh is used instead of the Mylar flap.

DETAILED DESCRIPTION Turning now to FIG. 1, a typical electrophotographic copying device is schematically shown in conjunction with an embodiment of the invention. The device has a control panel with power on switch 1, a stepdown counter 2, and copy counter dials 3. A cylindrical drum 11 is mounted for rotation on a shaft and a photoconductive element 12 comprising photoconductive insulating layer 13 and a conductive backing layer is mounted on the outer periphery of drum 11.

A variety of photoconductive materials are conventionally employed in the photoconductive insulating layer 13, for example, amorphous or vitreous selenium, selenium alloys with tellurium and arsenic, cadmium selenide, cadmium sulfide, and zinc oxide in a resin binder.

Preexposure corona unit 17 deposits an electrical charge of the desired polarity on the photoconductive material while it is maintained in the dark. Document 19 is held in place on a transparent plane 21 and an image of the document is projected onto the surface of the photoconductive insulating layer 13 by means of an illuminated scanning station 23 and optics 25. The photoconductor is discharged at 27 in the portions struck by the light to form a charged image corresponding to document 19. Drum 11 is rotated to bring the image to developer station 29 where finely divided charged toner particles are brought into contact with the charged image on the surface of photoconductive layer 13. The developer station can employ a variety of different developer means, as previously mentioned, for example, a cascade developer unit, powder cloud developer unit or magnetic brush developer unit. The developer station illustrated at 29 is a cascade developer unit wherein a two-component developer composition is caused to move across the surface of the charged image on the photoconductive layer 13. The developer composition comprises relatively large carrier particles and relatively small heat fixable marking particles of toner as described, for example, in U.S. Pat. No. 2,618,552. The toner particles are attracted to and cling to the charged areas of the photoconductive layer 13.

At station 31 the toned image on the surface of photoconductive layer 13 is transferred to a plain paper sheet or web 33 with the assistance of a transfer corona unit 35 which charges the paper to a polarity opposite to that of the toner particles so that it will attract the toner away from the surface of the photoconductive layer 13. The paper is then stripped from the photoconductive layer and passed to a heating unit 37 which acts to fuse and fix the toner image onto the paper.

A certain proportion of the toner particles remain on the photoconductive layer as a result of the fact that transfer efficiency to the paper is less than percent; and sometimes, because the length of paper copy sheet chosen is shorter than the length of the toned image on the drum, none of the toner is transferred from some image areas.

The rotating photoconductive layer is then passed into cleaning station 39 where it is contacted by a driven cylindrical cleaning and treatment brush 41 whose length corresponds approximately to the width of photoconductive layer 13 on the drum. Brush 41 is mounted in a housing 43 and an air flow in the direction shown by the arrows is provided by a vacuum means 45 to carry off the removed toner to a filter bag 47. A knockoff bar 49 aids in removing toner particles from the brush so that the brush remains relatively free from toner particles upon extended use. Mounted inside the brush is a cylindrical fluorescent lamp 55 (usually about 1 watt per inch) connected to a conventional electrical power source which is not shown.

Any suitable means can be used to provide radiation of the proper wave lengths to discharge the residual charge on photoconductive layer 13 as brush 41 removes the toner particles. Brush 41 comprises strips 57 of fabric pile material of polytetrafluoroethylene such as is described in a copending U.S. application of Ray L. Dueltgen and Carl A. Queener, Ser. No. 762,952, filed Sept. 26, 1968, and assigned to the assignee of this invention. Other materials can be used for the nap of the cleaning brush such as various types of furs, for example, beaver fur, gray fox fur, rabbit fur, and tiberlike synthetic materials such as nylon rayon or the like. The synthetic fibers are woven or knit into conventional backing layers, such as, for example, cotton or polypropylene and cemented in place at the backing layer by a coating of latex, The strips may be wound and secured to a transparent core of acrylic plastic. Other light-transmitting plastics or glass can be employed for the core material. Other methods of winding and securing to a core can also be employed. The total amount of light reaching the photoconductor can be controlled to a useful degree by the percentage of core area left uncovered between strips.

Prior Transfer Corona Station FIG. 2a illustrates a configuration of a prior transfer corona station including a base member 60 with transfer corona wires 62 and 63 connected to a high negative potential at terminal 64. FIG. 2 h simply illustrates a drum configuration for the OPC element and the prior transfer station 65 and indicates by arrows 66 and 67 the existence of maximum electrostatic pres sure due to the paper and catgut relationship. The catgut strings 68 are provided on the base member 60 in order to inhibit paper from entering the open transfer station. These are shown further in FIG. 20. With such an arrangement, the catgut strings will tend to establish visible streaks in the copy sheet as it passes the transfer station in solid copy areas. It is felt that the streaking condition encountered with prior devices is not due to contact between the catgut strings and the paper, or the catgut would show signs of wear. It has been concluded that the streaking is probably primarily due to electrostatic enhancement factors rather than the contact noted.

FIG. 3a illustrates some of the streaking conditions that may exist in such a prior device. With the arrangements according to the present invention, copies are much clearer and the streaking condition is practically eliminated as shown in FIGS. 31; and 3c.

Insulating Mats FIGS. 40 and 4b illustrate several configurationsthat are possible for insulating mats that have a high negative charge developed on them to repel the negatively charged paper and insure better pressure between the paper and the OPC drum element. The mat element, as can be seen in FIGS. 4a and 4b, is structured differently than the individual catgut strings discussed in connection with FIGS. 24, 2b, and 2c. FIGS. 4a and 4b simply illustrate two possibleconfigurations of insulating mats with potential applied to these mats from the-same source that providespotential tothe corona wires, such as corona wires 70 and 71 in FIG. 4a. Mat 75 includes a single centrally oriented portion 754 from which other elements extend at a slight angle to the direction of paper travel indicated by arrow 77; Mat 80, FIG. 4b, on the other hand, has three parallel arranged portions with angularly positioned, elements. extending from them again at an angular relationship with respect to the path of travel of they paper copy also indicated by arrow 77. The individual mats may be, of a Mylar, Dacron nylon or comparable material which is essentially an insulating material but which can acquire an electrostatic charge under appropriate conditions.

Transfer Station .with Insulating Flap A transfer station .85, FIGS. 5a, 5b, and 5c is provided .with an insulating flap member 86 that may beof a material such as Mylar. It is seen in these figures that.the Mylar, having a thickness of perhaps 2 mils, is .positioned-in such a manner I that it partially. covers the transfer corona station opening in which the transfer wires 87 and 88 are provided. The clearance is established to enable the highly charged ions from within the transfer station to reach the underneath side of paper sheet 33 as it traverses the station adjacentthe OPC' drum element 11 and photoconductor material 13. With the arrangement shown, the Mylar flap is concurrently charged to a high negative potential along with the'charging of the paper 33. This creates two effects. It creates a strong repelling field.

to the paper due to the high negative voltage on the flap and also creates an additional electrical field to. attract positive toner particles to the paper 33. With the arrangement shown, the streaking conditions in, FIG. 3a are eliminated. The leading and trailing edges of the copy also have a much better transfer and the copy quality is greatly improved. it has been found also that the total toner concentration of mix canbe lowered. while thesame optical density of characters with enhanced tone images is well maintained.

Due to the improved transfer of toner to the paper 33, there is less residual toner left on the surface of the photoconductor 13, thus improving the life of the cleaning filter bag. Also due to less residual toner, the overall life of the photoconductor l3 increment maybe improved. With improved optical density of images at lower toner concentration, less toner needs to be added percopy, that is, more copies may be produced with the same amount of toner in the system.

Transfer Station with lnsulating Mesh FIGS. 6a, 6b, and 6c illustrate a transfer station 90 having tive ions from the transfer wires-to approach the underneathside of paper 33 and establishthe necessary charging conditions to attract the positively charged toner from the photoconductor. surface 13. The insulating mesh attains a highly negative surface voltage, thus 'creating an additional. field approximately l0 to mils below the charged paper 33. The additional field has the etfect .of causing the paper 33 to be repelled from-the nylon mesh because of the existence of negative charges on both the paper and the mesh; and also, the high negativetcharge on the mesh below the paper helps the toner to be attracted from the photoconductor surfacel3 of drum 11. This is because the mesh charge adds to the transfer charge on the underneath side of paper 33 to enhance transfer. In a typical application, it has been found that a Dacron mesh with fibers, approximately 3.5 mils diameter and having a square opening of approximately 30 mils on a side, have enhanced the. transfer of the toner and elimination of streaks as well as thedropout conditionspreviously encountered in prior devices. As a result, overall copy quality has been greatly improved.

Since very similar results have been obtained using a flap of insulating material as .well as a mesh of insulating material, it is considered highly possible. that other configurations of materials, including flaps, meshes, screens of insulating material, and the like, may obtain similar results. Theprimary point offered by the additional transferfiap ormesh, or other structure, is that the high negative potential on the surface of this member in close proximity to the paperat the point of transfer insures that a better transfer of toner takes place andalso eliminates the streaking and dropout conditions previously encountered.

While the invention has been. particularly shown and described with reference to several preferred embodiments thereof,-it will be understood by those slcilled in the art that the foregoing and otherchanges in form and detail may be made therein without departing from the spirit and scope of the invention. i

What is claimed is:

l. Electrophotographic apparatus for producing copy documents from master documents and having facilities for scanning a master document, imaging a photoconductor element, developing said image, and relatively moving said photoconductor element and a copy document with respect to a transfer station to effect transfer of toner particles from said photoconductor element to said copy document, said transfer station including at least one element positioned adjacent the path of movement-of a said copy document, the improvement comprising:

an auxiliarymember associated with and mounted at said transfer station; andcharging meansat said transfer station for chargingtsaid copy document thereby establishing a primary transfer field at saidtransfer station that is effective to attract toner particles from said photoconductor surface to said copy document, said charging means being further operable to chargesaid auxiliary member. in order to establish an auxiliary field at said transfer station for urging said copy document into closer contact with said photoconductor element to insure that a slight distance exists between. said copy document and said at least one transfer .station element thereby minimizing frictional contactbetween said copy document and said transfer station.

2. The apparatus of claim 1 wherein:

said auxiliary member comprises a Mylar flap coextensive with the transferstation and partially covering the field generatingarea thereby enabling passage of charge particles to,a.copy document while concurrently charging said auxiliary member.

3. The apparatus of claim 2, further comprising;

meansfor positioning said Mylar flap in such a fashion with respect to the charging means at said transfer station that an intense field is established at an edge of said flap tending to repel'the transfer charge on the copy document proper thereby urgingsaid copy document with greater force toward said photoconductor element and enhancing the transfer of toner particles to the opposite surface of said copy document.

4. Theapparatus of claim 3, wherein:

said repelling transfer and auxiliary fieldsare of sufficiently high intensity than slight gap exists between a said copy document and said at least one transfer element at said transfer station, thereby minimizing :frictional contact between said copy document and; said at least one 8. The apparatus of claim 5, wherein: said mesh allows establishment of said primary transfer field through mesh openings for the purpose of effecting transfer of toner while concurrently charging said mesh grid in order to intensify the transfer operation by repelling the charge on said copy document as well as establishing a slight gap between said copy document and the transfer means thereby minimizing frictional contact between said copy document and said transfer station otherwise encountered during the transfer operation. 

1. Electrophotographic apparatus for produCing copy documents from master documents and having facilities for scanning a master document, imaging a photoconductor element, developing said image, and relatively moving said photoconductor element and a copy document with respect to a transfer station to effect transfer of toner particles from said photoconductor element to said copy document, said transfer station including at least one element positioned adjacent the path of movement of a said copy document, the improvement comprising: an auxiliary member associated with and mounted at said transfer station; and charging means at said transfer station for charging said copy document thereby establishing a primary transfer field at said transfer station that is effective to attract toner particles from said photoconductor surface to said copy document, said charging means being further operable to charge said auxiliary member in order to establish an auxiliary field at said transfer station for urging said copy document into closer contact with said photoconductor element to insure that a slight distance exists between said copy document and said at least one transfer station element thereby minimizing frictional contact between said copy document and said transfer station.
 2. The apparatus of claim 1 wherein: said auxiliary member comprises a Mylar flap coextensive with the transfer station and partially covering the field generating area thereby enabling passage of charge particles to a copy document while concurrently charging said auxiliary member.
 3. The apparatus of claim 2, further comprising: means for positioning said Mylar flap in such a fashion with respect to the charging means at said transfer station that an intense field is established at an edge of said flap tending to repel the transfer charge on the copy document proper, thereby urging said copy document with greater force toward said photoconductor element and enhancing the transfer of toner particles to the opposite surface of said copy document.
 4. The apparatus of claim 3, wherein: said repelling transfer and auxiliary fields are of sufficiently high intensity that a slight gap exists between a said copy document and said at least one transfer element at said transfer station, thereby minimizing frictional contact between said copy document and said at least one transfer element at said transfer station otherwise encountered during the transfer operation.
 5. The apparatus of claim 1, wherein: said auxiliary member is a mesh having a threaded grid pattern with threads positioned at approximately a 45* angle to the direction of movement of a said copy document.
 6. The apparatus of claim 5, wherein: said mesh is of a nylon material.
 7. The apparatus of claim 5, wherein: said mesh is a Dacron material.
 8. The apparatus of claim 5, wherein: said mesh allows establishment of said primary transfer field through mesh openings for the purpose of effecting transfer of toner while concurrently charging said mesh grid in order to intensify the transfer operation by repelling the charge on said copy document as well as establishing a slight gap between said copy document and the transfer means thereby minimizing frictional contact between said copy document and said transfer station otherwise encountered during the transfer operation. 